Vice President Mike Pence is already calling foul. It's a breach of presidential tradition and "very disappointing," Pence says, that former President Barack Obama will be blasting President Donald Trump and his administration in a series of rallies to support Democratic candidates running for congressional seats in midterm elections.

But Obama's in the fight, and he's committed to campaign for the next two months. Is this because the Republicans are strong in these contested districts and the Democrats vulnerable?